Practice, practice, practice. I can't formally play any instrument or read a lick of sheet music, so I find it to be an amazing skill to watch. Above Eddie said 10 hours of practice for every 1 hour on stage, and that basically hits the nail on the head. My dad was a professional performer, and he'd rehearse hours every day for 2 weeks straight before doing a show that lasted an hour and a half.

Cheers,
Deryk